With several recent refinery incidents resulting in unexpected plant shutdowns and even fatalities, more attention is being focused on safety and reliability programs. In this regard, how often should gas-monitoring instruments be tested and calibrated?
Answers
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17/09/2007
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KengYong Chan, Air Products & Chemicals, Inc, chanky@apci.com
The conventional approach on gas monitors testing or calibration is to follow the recommended frequency set by the gas monitors vendors or per gas monitor maintenance instructions. Typically, it ranges from 6 months to a year. There may be a subtle difference between testing and calibration; calibration normally is intended for accuracy (correct drifting or offset) while testing is intended for function check.
Also, it is important to distinguish these gas monitors (fixed type; not portable) as having either a safety impact or product quality / reliability impact. Gas monitors which have safety impact are also typically classified as critical safety. These gas monitors can also be part of the safety instrumented function (such that it triggers a valve action or shut down an equipment such as a compressor or pump). It is important to ensure that the entire safety instrumented function circuit (gas moniotr, valve or equipment action etc) works per design during testing. Categorization of these gas monitors allow one to understand the significance or impact when these gas monitors fail to function on demand.
The proof test frequency for these gas monitors (safety or reliability impact) should be according to
1) the vendor's recommended specifications for that specific model of gas monitors, or
2) actual site experiences of the gas monitor performance, or - there could be more frequent drifting or failure associated with a particular model of gas monitor at the site due to the operating/weather conditions that warrant more frequent calibration or testing.
3) results from Fault Tree Analysis, - the fault tree analysis may indicate a higher proof test frequency is required to improve the probability of failure on demand, such that it can meet the overall targeted frequency.
For a single facility with hundreds of gas monitors (e.g. refinery), a program needs to be developed to track and measure the performance of these gas monitors (safety or reliability), and at the same time, alert overdues. The performance (failure rate frequency, reasons for failure etc) of these monitors will be useful for further analysis and improvement (extending test frequency etc). Safety critical gas monitors should be diligently monitored, calibrated and tracked as these are often not on demand.
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24/07/2007
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Matt Thiel, Industrial Scientific Corporation, d385d5b9-8303-44f7-adc7-bab7c8552ed9
Gas monitoring instrumentation should be treated just like any other piece of lifesaving equipment, and be tested and calibrated on a regular basis. The safest approach to testing gas monitors is to function, or “bump”, test them prior to each day’s use. A function test consists of exposing each sensor in the gas monitor to a known concentration of gas(es) in excess of the lowest alarm set-point. The instrument should respond to the gas concentration by going into alarm. If the sensors do not respond to the applied gas, the instrument should be calibrated before it is used. The frequency of testing is best determined by company policies or local regulations that may mandate specific testing intervals.
Instrument calibration is just as important as testing. Calibration of instruments has become a simple and sometimes automated process. The process consists of exposing the instrument sensors to a known concentration of gas(es) and making the appropriate response adjustments to ensure the instrument is reading accurately. Typically, the instrument does these adjustments automatically without any user intervention. Calibration should be performed every 30 days. Company policy or local regulatory agencies may dictate either less or more frequent calibrations. One thing to consider when it comes to the testing and calibration of gas-monitoring equipment is instrument use and abuse experienced in the field. These instruments are used in some of the harshest and dirtiest workplace conditions. Sensors can easily become damaged or blocked by dirt and debris. If gas cannot diffuse into the sensor, it will not respond to it. Visual inspection alone cannot detect blocked sensors, only function testing guarantees that the sensor can still detect gas.
Gas exposure is another field condition that can affect frequency of testing and calibration. Sensors can become damaged due to exposure to certain gases. These gases, in either high or low concentrations, could damage the electrodes or hinder or inhibit the sensing elements from working properly. Calibration is needed to readjust the instrument’s accuracy after such an exposure occurs. For catalytic combustible sensors, function testing is a method of burning off some of the inhibitive materials that could build up on the sensor’s pellistor.
Operator use is the final element in determining calibration and bump testing. Each individual uses instruments differently. One operator may take extreme care of the instrument, while another may physically abuse it. When gas monitors experience high levels of shock or abuse, they should be tested and calibrated more frequently to ensure the integrity of the sensors. Testing and calibration are standard maintenance items for all gas monitors. They should not be taken lightly or ignored. There are a number of different regulatory agencies that make recommendations for the frequency and methods of testing and calibration. Local regulations should be reviewed to ensure they are being met in their respective areas. All of these regulations and standards are put in place for one reason: worker safety. Worker safety should be the number one priority when establishing testing and calibration procedures. Ask yourself if you would risk your life, or the life of one of your workers, by using a monitor that has not been tested or calibrated.
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